b
(b) The distance between two points i.e. path difference between them
$\Delta = \frac{\lambda }{{2\pi }} \times \phi = \frac{\lambda }{{2\pi }} \times \frac{\pi }{3} = \frac{\lambda }{6} = \frac{v}{{6n}}$ $(\,\because \,\,v = n\lambda )$
$\Delta = \frac{{360}}{{6 \times 500}} = 0.12\,m = 12\,cm$